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Past Seminars

A Grand View of Self-organization in an Open System

Tetsuya Sato: NIFS

A grand view of self-organization is constructed from the results of our various simulation studies such as generation of dipole magnetic field, (so-called geodynamo), coagulation of fine grains in plasmas, crystallization of polymer chains, solar loops and flares, merging of spheromoks, magnetohydrodynamic self-organization.

Space Weather

Dan Baker: LASP - University of Colorado at Boulder

The space environment is known to have significant effects upon human technological systems. The broad solar-magnetospheric-ionospheric set of variations having such adverse impacts on human endeavors are called "space weather". In this talk I will present information about a number of the most well-known and well-documented elements of space weather with a focus on satellite operational anomalies. The physical causes of such problems and possible ways of averting problems in the future will also be described. Several recent examples of space weather impacts will be presented including the recent failure of the AT&T Telstar 401 satellite on 11 January 1997.

Dynamics of Got Filaments in a Tokamak Plasma

Hans De Sterck: NCAR-HAO

Recent Thomson Scattering measurements at the RTP Tokamak of the FOM-Insitute for Plasma Physics (The Netherlands) reveal the existence of many hot plasma filaments in the center of an externally heated plasma. These observations break down the old paradigm that describes a Tokamak plasma as a nest of closed magnetic flux surfaces. The plasma filamentation may account for many unexplained transport and confinement properties. The measurements however are very incomplete and leave open many questions on the formation, structure, lifetime and dynamics of the filaments. In this talk I will discuss numerical simulations which model the structure, lifetime and dynamics of hot plasma filaments in the magnetohydrodynamic (MHD) model, using a two-dimensional resistive MHD code. I will give a brief overview of the filamentation measurements, of the MHD model and of the finite volume numerical technique which is used. This will be foll owed by a more extensive discussion of the simulation results.

Large-scale Gyrokinetic Simulation of Tokamak Plasma Turbulence

Scott E. Parker: CIPS - University of Colorado at Boulder

Three-dimensional kinetic turbulence simulations are producing fluctuation spectra and transport levels similar to experimental results for the first time ever. This has occurred due to developments in reduced equations (gyrokinetic formalism), numerical methods (low-noise delta-f method) and the enormous gains in massively parallel computing. This talk will give an overview of these fairly recent developments.

Observation of Spherical Focus in an Electron Penning Trap

Travis Mitchell: NIST

We produce simultaneously dense and well-confined nonneutral plasmas by spherical focusing. A small (3mm radius) Penning trap has low-energy electrons injected at a single pole of the sphere. Precisely when the trap parameters are adjusted to produce a spherical well, the system self-organizes into a spherical state, through a bootstrapping mechanism which produces a hysteresis. Additional confirmation of the dense spherical focus is provided by electrons scattered by the central core. Core densities up to 35 times the Brillouin density have been inferred from the data.

A New Method for Designing Particle Accelerators with Larger Stable Region

Weishi Wan: CIPS - University of Colorado at Boulder

We have previously proposed a method for reducing chaos in four-dimensional symplectic maps, which describe the motion of charged particles in accelerators. The method relies on solving for parameter values at which the linear stability factors of the map have the values corresponding to integrability. We suggest that this method be applied to accelerator lattices in order to increase the volume of stable region in the phase space. We have now implemented a computational scheme for the practical application of this method to accelerator lattices. The Advanced Light Source (ALS) lattice is used as an example to test this method. Preliminary results show substantial increase of the volume of stable region which implies the validity of the method.

Stochastic Broadening in 1D Self-consistent Current Sheets Models

James Harold: Space Science Institute

The Earth's magnetotail current sheet has received a significant amount of attention since its discovery, due to its suspected role in the storage and subsequent release of magnetic energy associated with geomagnetic substorms. The weakly magnetized nature of the current sheet region demands that modeling efforts retain the particle physics, so that fluid based models cannot be used to accurately model this region.

We report on the results of simple, 1D particle based equilibrium models of the current sheet. In particular, we discuss the importance of particle stochasticity in determining the gross current sheet structure. We have found that small numbers of stochastic particles can lead to significant thickening of the current sheet, with potential consequences for current sheet instability theories. In addition, the quasi-trapped nature of these particles can lead to force balance failure, so that in certain parameter regimes (particularly in the near Earth, dipolar region) thin, 1D current sheet solutions do not exist.

Bedros B. Afeyan: UC-Davis

Various Models of the Parametric Decay Instability (PDI) try to address the effects of plasma inhomogeneity and nonlinearity simultaneously in order to assess the evolution of this instability in ionospheric and laser produced plasmas. David Newman and I have been cooperating on developing analytical and numerical schemes which can attack these issues from various mutually complementary points of view. This seminar will report on the progress we have made so far including comparisons between our various results. A discussion of our plans for future work will also be presented. In the context of laser plasma interactions, there is great interest in reduced descriptions that capture the macroscopic physics on length and time scales of interest. PDI, OTSI (Oscillating Two Stream instability) and related phenomena at or near the critical density are excellent testbeds for the study of all parametric instabilities which involve nonlinear interactions between plasma modes. Secondary decay processes, Zakharovia and beyond are the pressing issues waiting to be tackled.

NIMROD: Nonideal Magnetohydrodynamics with Rotation, an Open Discussion Code Development Project

Carl Sovinec: LANL

A new computational tool for plasma physics research is being developed with support from the Office of Fusion Energy. The purpose of the project is twofold. First, there is a need for improved modeling of low-frequency behavior in tokamaks, where a soft beta limit has been observed to inhibit experimental performance. Second, the project itself is an experiment in modern code development. The team uses concurrent engineering with Quality Function Deployment to direct and coordinate developers who are scattered across the U.S. Technical highlights include a general two-fluid formulation that retains all terms in the cold ion and electron momentum density equations. An implicit Ampere's law ensures numerical stability at large time-steps, though the equations support high-frequency normal modes at small time-steps. Energy equations for the two species are advanced separately with semi-implicit operators to stabilize high wavenumber sound waves, and a variety of closure schemes may be used. The spatial representation features finite element discretization in the poloidal plane and Fourier components in the toroidal direction. This will permit efficient modeling of complicated experimental cross sections for a variety of experiments.

Review of APS Posters

Scott H. Robertson: CIPS - University of Colorado at Boulder

Saturday November 02 1996, 1:00 - 2:00pm in Gamow Tower, Room F931

Precision Control of E x B Rotation of Non-neutral Plasmas in Penning Traps

Pei Huang: NIST

Non-neutral plasmas contain charges with only one sign, and are typically confined by static electric and magnetic fields in Penning traps. Except for a global cross-field rotation, these plasmas follow the dynamics of One Component Plasmas and are unique in that they can relax to global thermal equilibrium while confined, allowing incisive quantative measurements of fundamental processes for direct comparison with theories. I will give a brief introduction to the subject and then concentrate on the NIST experiment using laser-cooled Be+ plasmas and crystals. Here, we have been able to stabilize and control the global ExB rotation of up to 10^6 Be+ ions with an externally applied electric asymmetry rotating in the same direction as the ions. Laser induced fluorescent images show that the aspect ratio of these steady-state spheroidal plasmas, which determines the equilibrium density and rotation frequency, can be varied by gradually changing the frequency of the external drive. Furthermore, accurate photon correlation measurements on Bragg scattering patterns from crystals indicate that the lattice structure can be stable for long periods of time (up to one hour), and they can be precisely phase locked to the applied field without any slip. This technique enables precise control of the plasma properties which is important for trapped ion frequency standards, and may also be utilized in other Penning trap experiments such as the syntheses of antihydrogen.

Femtosecond Laser-produced Plasma Diagnostics

Tracy S. Clement: JILA

The interaction of ultrashort, high-intensity pulses with gas targets is a field of growing interest. Highly-ionized atmospheric-density plasmas are a potential source of coherent vuv and x-ray light , as well as a potential source for charged-particle acceleration. Creation of such plasmas via ionization by intense, femtosecond pulses holds promise for the precise control of the initial plasma conditions which are critical to these applications. At the same time, new quantitative experimental diagnostics compatible with high gas density and ultrafast time scales are needed to measure the ionization and subsequent plasma dynamics which give rise to these conditions. We have developed a technique for measuring atomic ionization rates in the presence of high-intensity laser light based on the direct measurement of the phase change of an optical pulse. Knowledge of these rates is important in many areas of strong-field physics, most notably in the laser-fusion arena. We use this technique to study the ionization dynamics of atmospheric-pressure noble gases interacting with high-intensity, femtosecond laser pulses.

Precipitation Regions in the Ionosphere and Their Relations to Magnetospheric Source Regions

Terry Onsager: NOAA-SEC

Low-altitude particle precipitation in the ionosphere provides an important measurement of the distant magnetospheric regions that are connected to the ionosphere by the magnetic field. From spacecraft measurements at high latitudes in the dayside ionosphere we are able to investigate the distant magnetopause, including the effects of magnetic reconnection. In the nightside ionosphere we are able to investigate the distant plasma sheet extending from the Earth to a downtail reconnection site, and to study its temporal and spatial variability. This presentation will illustrate the low-altitude particle signatures of the magnetospheric source regions, and describe techniques used to provide quantitative estimates of the processes occurring in these distant regions, including particle energization and transport across the dayside magnetopause and particle energization near the distant magnetotail reconnection site and in the tail current sheet.

New Magnetic Configurations with Good Plasma Confinement

Svetlana G. Shasharina and John R. Cary: CIPS - University of Colorado at Boulder

I will present a new magnetic configuration for plasma confinement. I will explain why it is new and why it has a better confinement. This system turns out to be omnigenous (meaning that the radial displacement of particle orbits is zero on average) and far from being quasihelical.

Waves in the E-region Ionosphere - Simulations, Movies and Theory

Meers Oppenheim: CIPS - University of Colorado at Boulder

The electrojet is an ionospheric current strong enough enough to deflect a compass needle. This current flows along the Earth's magnetic equator and in the auroral ionosphere within the {\em E}-region (90 - 120 km in altitude). Two plasma instabilities disrupt the flow of the electrojet current: the modified two-stream (Farley-Buneman) and the gradient-drift instabilities. I shall argue that both these instabilities nonlinearly drive D.C. currents in the {\em E}-region ionosphere. These currents flow parallel to, and with a comparable magnitude to, the fundamental Pederson current. Hence, wave-driven currents act to discharge the electrojet, effectively reducing the resistivity of the {E}-region. This talk will review the physics of E-region waves, show a number of results from simulations of the two-stream instability, describe the nonlinear behavior leading to DC currents, and discuss a few implications of this nonlinear current.

New Results on the Destabilization of Thin Current Sheets: Three-Dimensional Kinetic "Tearing"

Joerg Buechner: Max Planck Institute for Extraterrestrial Physics

A destabilization of thin current sheets in the near-Earth's magnetotail was suggested to cause the onset of magnetospheric substorms and reconnection in space plasma in general. Current instabilities as well as two-dimensional tearing instabilities were supposed to be the physical mechanism of this process. Unfortunately, neither did the possible current instabilities appear to provide enough "anomalous" resisitivity nor did the two-dimensional tearing mode proof to grow for typical initial current sheet configurations. Considering the current sheet in three dimensions fully kinetically we have now obtained the conditions for the current sheet destabilization. They appear to be essentially three-dimensional, combining current instabilies and sheet tearing on the kinetic level of particle dynamics. The three-dimensional unstable sheet decay is demonstrated by means of simulation runs carried out with our newly developed 3D electromagnetic, fully kinetic PIC code GISMO.

Nonlinear Growth Rates for Longitudinal Plasma Turbulence

Isidoros Doxas: CIPS - University of Colorado at Boulder

We have observed non-quasilinear diffusion in the regime t_ac/t_d[[1, where t_ac is the autocorrelation time and t_d the difusion time, in contradiction to accepted theory. The diffusion enhancement was observed in the regime t_g/t_rb]]1, where t_g is the linear growth time and t_rb the resonance broadening time, in agreement with the theoretical prediction of Laval and Pesme (1980, 1983, 1984). If we define the overlap parameter to be A=(pi/2)^2 (2 d_r/d_v)^2 where d_r is the mode resonance width and d_v the intermode separation, then a high overlap parameter is an approximation to a continuous spectrum. The enhancement is seen in spectra initialized with high overlap paramenter (A>200), implying that a continuous spectrum might also produce enhanced transport in the nonlinear regime. A possible mechanism for the enhanced diffusion (spontaneous spectrum discretization) is discussed.

The Earth's Electron Foreshock - A Beam-plasma "Laboratory" in Space

David L. Newman: CIPS - University of Colorado at Boulder

The Earth's Foreshock comprises that region of the solar wind outside---but magnetically connected to---the bow shock. The interaction of the solar wind with the Earth's bow shock provides an electron acceleration mechanism (near the the point of tangency of the interplanetary magnetic field with the shock), as well as a natural velocity filter for the accelerated electrons. Together, these two processes can produce electron distributions in the Foreshock that are unstable to the generation of Langmuir waves. While there already exist a wealth of satellite measurements of Langmuir waves in the Foreshock, the unstable electron distributions responsible for these waves have, paradoxically, been observed far less frequently. In order to better understand the mutual (nonlinear) interaction of the electron distribution with the waves (including the influence of wave-wave interactions), we have undertaken a series of 1-D kinetic Vlasov Equation simulations for relevant Foreshock parameters. These simulations demonstrate the subtle interplay between the various nonlinear processes, and shed light on the origin of an anti-beamward plateau observed in some measured distributions.